The relative molecular mass has a positive influencing effect on the antibacterial activity of chitosan derivatives as chitosan and its derivatives have road-spectrum antimicrobial properties. Several different molecular weights of water-soluble chitosan oligosaccharides were used to conduct antibacterial experiments on staphyloccocus aureus，escherichia coli and aspergillus niger, and the effects of relative molecular mass on the bacteriostatic activity of chitooligosaccharides were explored. The experimental results showed that relative molecular mass had certain effects on the antibacterial activity but with no regularity, and the water-soluble chitooligosaccharides with molecular weight of 2.00×103 performed better in the antibacterial activity. Chitosan and its different molecular weight of water-soluble chitooligosaccharides showed the best antibacterial activity against staphylococcus aureus, followed by aspergillus niger and the poorest antibacterial activity against escherichia coli.